Field-effect transistor, method for producing same, display element, display device, and system

ABSTRACT

(Object) To miniaturize a field-effect transistor. (Means of Achieving the Object) A field-effect transistor includes a semiconductor film formed on a base, a gate insulating film formed on a part of the semiconductor film, a gate electrode formed on the gate insulating film, and a source electrode and a drain electrode formed in contact with the semiconductor film, wherein a thickness of the source electrode and the drain electrode is smaller than a thickness of the gate insulating film, and the gate insulating film includes a region that is not in contact with the source electrode or the drain electrode.

TECHNICAL FIELD

The disclosures herein generally relate to a field-effect transistor, amethod for producing the same, a display element, a display device, anda system.

BACKGROUND ART

Field-effect transistors (FETs) have a low gate current and a flatstructure. Therefore, the FETs can be easily produced and can also beeasily integrated as compared to bipolar transistors. For these reasons,the field-effect transistors are widely used in integrated circuits usedin existing electronic devices.

In such field-effect transistors, silicon, oxide semiconductors, andorganic semiconductors are used for semiconductor films. Examples ofsuch field-effect transistors include a field-effect transistor using anoxide semiconductor film with a self-aligned structure. The field-effecttransistor has a structure in which a semiconductor film is covered byan interlayer insulating layer, contact holes are formed in theinterlayer insulating layer, and a source electrode and a drainelectrode formed on the insulating layer are connected to a sourceregion and a drain region through the contact holes. Also, the oxidesemiconductor film of the field-effect transistor is provided with achannel forming region and a low resistance region that has lowerresistance than that of the channel forming region. Further, an impurityregion is formed between the channel forming region and the lowresistance region (see Patent Document 1, for example).

CITATION LIST Patent Literature

[NPL 1] Japanese Unexamined Patent Application Publication No.2013-175710

SUMMARY OF INVENTION Technical Problem

However, the structure of the above-described field-effect transistor isrequired to allow for variations in positions where contact holes, asource electrode, and a drain electrode are formed. Therefore, thestructure of the above-described field-effect transistor is not suitablefor miniaturization. Further, given that the impurity region is formedbetween the channel forming region and the low resistance region, theabove-described field-effect transistor is not suitable forminiaturization.

In view of the above, it is an object of an embodiment of the presentinvention to miniaturize a field-effect transistor.

Solution to Problem

A field-effect transistor includes a semiconductor film formed on abase, a gate insulating film formed on a part of the semiconductor film,a gate electrode formed on the gate insulating film, and a sourceelectrode and a drain electrode formed in contact with the semiconductorfilm, wherein a thickness of the source electrode and the drainelectrode is smaller than a thickness of the gate insulating film, andthe gate insulating film includes a region that is not in contact withthe source electrode or the drain electrode.

Advantageous Effects of Invention

According to at least one embodiment of the present disclosures, afield-effect transistor can be miniaturized.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1A is a diagram illustrating a field-effect transistor of a firstembodiment;

FIG. 1B is a diagram illustrating the field-effect transistor of thefirst embodiment;

FIG. 2A is a diagram (part 1) illustrating a process for producing thefield-effect transistor of the first embodiment;

FIG. 2B is a diagram (part 1) illustrating a process for producing thefield-effect transistor of the first embodiment;

FIG. 2C is a diagram (part 1) illustrating a process for producing thefield-effect transistor of the first embodiment;

FIG. 2D is a diagram (part 1) illustrating a process for producing thefield-effect transistor of the first embodiment;

FIG. 3A is a diagram (part 2) illustrating a process for producing thefield-effect transistor of the first embodiment;

FIG. 3B is a diagram (part 2) illustrating a process for producing thefield-effect transistor of the first embodiment;

FIG. 3C is a diagram (part 2) illustrating a process for producing thefield-effect transistor of the first embodiment;

FIG. 4 is a cross-sectional view illustrating a field-effect transistorof a second embodiment;

FIG. 5 is a cross-sectional view illustrating a field-effect transistorof a third embodiment;

FIG. 6A is a diagram illustrating a process for producing thefield-effect transistor of the third embodiment;

FIG. 6B is a diagram illustrating a process for producing thefield-effect transistor of the third embodiment;

FIG. 6C is a diagram illustrating a process for producing thefield-effect transistor of the third embodiment;

FIG. 7 is a cross-sectional view illustrating a field-effect transistorof a fourth embodiment;

FIG. 8A is a diagram illustrating a process for producing thefield-effect transistor of the fourth embodiment;

FIG. 8B is a diagram illustrating a process for producing thefield-effect transistor of the fourth embodiment;

FIG. 8C is a diagram illustrating a process for producing thefield-effect transistor of the fourth embodiment;

FIG. 8D is a diagram illustrating a process for producing thefield-effect transistor of the fourth embodiment;

FIG. 9 is a cross-sectional view illustrating a field-effect transistorof a fifth embodiment;

FIG. 10 is a cross-sectional view illustrating a field-effect transistorof a sixth embodiment;

FIG. 11 is a diagram illustrating characteristics of a field-effecttransistor produced in Example 1;

FIG. 12 is a block diagram illustrating a configuration of a televisionapparatus of a seventh embodiment;

FIG. 13 is an explanatory diagram (part 1) of the television apparatusof the seventh embodiment;

FIG. 14 is an explanatory diagram (part 2) of the television apparatusof the seventh embodiment;

FIG. 15 is an explanatory diagram (part 3) of the television apparatusof the seventh embodiment;

FIG. 16 is an explanatory diagram of a display element of the seventhembodiment;

FIG. 17 is an explanatory diagram of an organic electroluminescent (EL)element of the seventh embodiment;

FIG. 18 is an explanatory diagram (part 4) of the television apparatusof the seventh embodiment;

FIG. 19 is an explanatory diagram (part 1) of another display element ofthe seventh embodiment; and

FIG. 20 is an explanatory diagram (part 2) of the another displayelement of the seventh embodiment.

DESCRIPTION OF EMBODIMENTS

In the following, embodiments of the present invention will be describedwith reference to the drawings.

In the drawings, the same elements are indicated by the same referencenumerals and a duplicate description thereof may be omitted.

First Embodiment

Configuration of Field-Effect Transistor

FIGS. 1A and 1B are diagrams illustrating a field-effect transistor of afirst embodiment. FIG. 1A is a cross-sectional view and FIG. 1B is aplan view. FIG. 1A illustrates a vertical sectional view taken alongline A-A of FIG. 1B. For convenience of explanation, some elementsillustrated in the plan view of FIG. 1B are indicated by the samehatching as that used in the cross-sectional view of FIG. 1A.

Referring to FIGS. 1A and 1B, a field-effect transistor 10 is atop-gate/top-contact field-effect transistor that includes a base 11, asemiconductor film 12, a gate insulating film 13, a gate electrode 14, asource electrode 15, a drain electrode 16, and a gate electrode coveringlayer 17. The field-effect transistor 10 may be a topgate/bottom-contactfield-effect transistor. The field-effect transistor 10 is a typicalexample of a semiconductor device.

In the present embodiment, for convenience, the gate electrode coveringlayer 17 side is represented as an upper side or one side, and the base11 side is represented as a lower side or the other side. Also, asurface of the respective elements on the gate electrode covering layer17 side is represented as an upper surface or one surface and a surfaceof the respective elements on the base 11 side is represented as a lowersurface or the other surface. However, the field-effect transistor 10can be used upside down or can be disposed at any angle. Also, a planview refers to viewing an object from an upper surface of the base 11 ina normal direction (z-axis direction). A planar shape refers to a shapeof an object when viewed from the upper surface of the base 11 in thenormal direction (z-axis direction). In addition, a vertical sectionrefers to a cross section of the respective elements on the base 11taken in a lamination direction. A transverse section refers to a crosssection of the respective elements on the base 11 taken in a directionperpendicular to the lamination direction (direction parallel to theupper surface of the base 11).

In the field-effect transistor 10, the semiconductor film 12 is formedin a predetermined region on the insulating base 11. The gate insulatingfilm 13 is formed in a predetermined region on the semiconductor film12. Also, the gate electrode 14 having the same pattern as that of thegate insulating film 13 is formed on the gate insulating film 13.Further, the source electrode 15 and the drain electrode 16 covering thebase 11 and the semiconductor film 12 are formed with the gateinsulating film 13 being interposed between the source electrode 15 andthe drain electrode 16, such that a channel is formed in thesemiconductor film 12. Further, the gate electrode covering layer 17 isformed on the gate electrode 14.

As used herein, the same pattern as that of the gate insulating filmrefers to a pattern in which the gate electrode substantially overlapsthe gate insulating film in a plan view. Also, substantially overlappingincludes, of course, a case in which the gate insulating film and thegate electrode have the same shape, and also includes, as will bedescribed below, a case in which an outer edge portion of an lowersurface of the gate electrode protrudes a few hundred nm from theperiphery of an upper surface of the gate insulating film and a case inwhich an outer edge portion of the upper surface of the gate insulatingfilm protrudes a few hundred nm from the periphery of the lower surfaceof the gate electrode, for example. Hereinafter, the respective elementsof the field-effect transistor 10 will be described in detail.

The base 11 is an insulating member on which the semiconductor film 12is formed. A shape, a structure, and a size of the base 11 are notparticularly limited and may be appropriately selected depending on thepurpose. By way of example, in FIGS. 1A and 1B, the planar shape of thebase 11 is formed in an approximately square shape.

A material of the base 11 is not particularly limited and may beappropriately selected depending on the purpose. For example, a glassbase, a plastic base, and the like may be used. The glass base is notparticularly limited and may be appropriately selected depending on thepurpose. Examples of the glass base include non-alkali glass and silicaglass.

The plastic base is not particularly limited and may be appropriatelyselected depending on the purpose. Examples of the plastic base includepolycarbonate (PC), polyimide (PI), polyethylene terephthalate (PET),and polyethylene naphthalate (PEN).

The semiconductor film 12 is formed in a predetermined region of thebase 11. A shape, a structure, and a size of the semiconductor film 12are not particularly limited and may be appropriately selected dependingon the purpose. By way of example, in FIGS. 1A and 1B, the planar shapeof the semiconductor film 12 is formed in a rectangular shape, with thelonger side being in the x-axis direction. The semiconductor film 12located between the source electrode 15 and the drain electrode 16serves as a channel region. An average thickness of the semiconductorfilm 12 is not particularly limited and may be appropriately selecteddepending on the purpose, but is preferably 5 nm to 1 μm and morepreferably 10 nm to 0.5 μm.

A material of the semiconductor film 12 is not particularly limited andmay be appropriately selected depending on the purpose. Examples of thematerial include organic semiconductors such as polycrystalline silicon(p-Si), amorphous silicon (a-Si), an oxide semiconductor, and pentacene.Of them, an oxide semiconductor is preferably used in terms of stabilityof an interface with the gate insulating film 13.

As an oxide semiconductor constituting the semiconductor film 12, ann-type oxide semiconductor can be used. The n-type oxide semiconductoris not particularly limited and may be appropriately selected dependingon the purpose. Preferably, the n-type oxide semiconductor includes atleast any one of indium (In), Zn, tin (Sn), and Ti, and also includes analkaline earth element or a rare earth element. Preferably, the n-typeoxide semiconductor includes In and also includes an alkaline earthelement or a rare earth element.

Examples of the alkaline earth element include beryllium (Be), magnesium(Mg), calcium (Ca), strontium (Sr), barium (Ba), and radium (Ra).

Examples of the rare earth element include scandium (Sc), yttrium (Y),lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd),promethium (Pm), samarium (Sm), europium (Eu), gadolinium (Gd), terbium(Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm),ytterbium (Yb), and lutetium(Lu).

An electron carrier density of indium oxide changes by approximately10¹⁸ cm⁻³ to 10²⁰ cm⁻³ depending on the amount of oxygen defects. Theindium oxide tends to have oxygen defects. Therefore, unintentionaloxygen defects may be produced during a post-process after asemiconductor film containing an oxide is formed. The oxide ispreferably formed mainly of two metals that are indium and an alkalineearth element or a rare earth element, both of which are easier to bondwith oxygen than indium. This makes it possible to easily control acomposition while preventing unintentional oxygen defects from beingproduced. Accordingly, the electron carrier density can also be properlycontrolled.

Also, the n-type oxide semiconductor constituting the semiconductor film12 undergoes substitutional doping with at least one dopant selectedfrom a divalent cation, a trivalent cation, a tetravalent cation, apentavalent cation, a hexavalent cation, a heptavalent cation, and anoctavalent cation. Preferably, a valence of the dopant may be greaterthan a valence of a metal ion (other than the dopant) constituting then-type oxide semiconductor. Substitutional doping is also referred to asn-type doping.

The gate insulating film 13 is provided between a part of semiconductorfilm 12 and the gate electrode 14. The gate insulating film 13 includesa region that is not in contact with the source electrode 15 or thedrain electrode 16. A shape, a structure, and a size of the gateinsulating film 13 are not particularly limited and may be appropriatelyselected depending on the purpose. By way of example, in FIGS. 1A and1B, the planar shape of the gate insulating film 13 is formed in arectangular shape, with the longer side being in the y-axis direction. Apart of the gate insulating film 13 extends from an upper surface of thesemiconductor film 12 in the y-axis direction and is formed directly onthe base 11.

The gate insulating film 13 is a layer for insulating the gate electrode14, the semiconductor film 12, the source electrode 15, and the drainelectrode 16 from one another. An average thickness of the gateinsulating film 13 is not particularly limited and may be appropriatelyselected depending on the purpose, but is preferably 50 nm to 1000 nmand more preferably 100 nm to 500 nm.

For example, the gate insulating film 13 is an oxide film. The oxidefilm contains a Group A element that is an alkaline earth metal and aGroup B element that is at least one of gallium (Ga), scandium (Sc),yttrium (Y), and a lanthanoid. The oxide film preferably contains aGroup C element that is at least one of Zr (zirconium) and Hf (hafnium),and further contains other components as necessary. The oxide film mayinclude one alkaline earth metal element or may include two or morealkaline earth metal elements.

Examples of a lanthanoid include lanthanum (La), cerium (Ce),praseodymium (Pr), neodymium (Nd), promethium (Pm), samarium (Sm),europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium(Ho), erbium (Er), thulium (Tm), ytterbium (Yb), and lutetium (Lu).

The oxide film contains a paraelectric amorphous oxide or is preferablyformed of a paraelectric amorphous oxide. A paraelectric amorphous oxideis stable in the atmosphere and can stably form an amorphous structurein a wide range of compositions. A crystal may be included in a part ofthe oxide film.

Alkaline earth oxides tend to react with moisture or carbon dioxide inthe atmosphere and are easily converted into hydroxides or carbonates.Therefore, such alkaline earth oxides alone are not suitable for use inelectronic devices. Further, simple oxides such as of lanthanoidsexcluding Ga, Sc, Y, and Ce tend to be crystallized and cause a leakagecurrent. However, oxides containing an alkaline earth metal and alanthanoid excluding Ga, Sc, Y, and Ce are stable in the atmosphere andcan form an amorphous film in a wide range of compositions. Amonglanthanoid elements, Ce specifically becomes tetravalent, and forms acrystal having a perovskite structure together with the alkaline earthmetal. Therefore, in order to obtain an amorphous phase, a lanthanoidexcluding Ce is desired.

Crystalline phases such as a spinel structure exist for oxidescontaining an alkaline earth metal and Ga. However, these crystals arenot precipitated unless the temperature is significantly high(generally, at 1,000° C. or more), as compared to crystals having aperovskite structure. Also, no report has been presented regarding astable crystalline phase for oxides containing an alkaline earth metaland a lanthanoid excluding Sc, Y, and Ce. Crystals are rarelyprecipitated from the amorphous phase even after the post-process at ahigh temperature. In addition, an amorphous phase becomes more stablewhen the oxides containing the alkaline earth metal and a lanthanoidexcluding Ga, Sc, Y, and Ce are formed of three of more metal elements.

The content of each element included in the oxide film is notspecifically limited. However, the oxide film preferably includes metalelements selected from respective element groups so as to form acomposition that can maintain a stable amorphous state.

In order to make a film having a high dielectric constant, compositionratios of elements such as Ba, Sr, Lu, and La are preferably increased.

Because the oxide film of the present embodiment can form an amorphousfilm in a wide range of compositions, the physical properties can alsobe widely controlled. For example, a dielectric constant of the oxidefilm of the present embodiment is generally approximately 6 to 20 and issufficiently high as compared to that of SiO₂. However, by selecting acomposition, the dielectric constant can be adjusted to an appropriatevalue according to purpose of use.

Further, a coefficient of thermal expansion for the oxide film of thepresent embodiment is equivalent to a coefficient of thermal expansionfor a general wiring material or a semiconductor material, which is 10⁻⁶to 10⁻⁵. Therefore, as compared to SiO₂ having a coefficient of thermalexpansion of 10⁻⁷, the oxide film of the present embodiment rarely hasproblems such as a peeling of a film even after a heating process isrepeatedly performed. In particular, with oxide semiconductors such asaIGZO, a favorable interface is formed.

Therefore, a high-performance semiconductor device can be provided byusing the oxide film of the present embodiment.

However, the gate insulating film 13 is not limited to the oxide filmthat contains at least a Group A element and a Group B element andpreferably contains a Group C element. For example, the gate insulatingfilm 13 may be an oxide film that contains Si and an alkaline earthmetal. Further, the gate insulating film 13 may be a film formed ofSiO₂, SiN, SiON, or Al₂O₃, for example.

The gate electrode 14 is formed on the gate insulating film 13. The gateelectrode 14 is an electrode that applies a gate voltage. The gateelectrode 14 is disposed facing the semiconductor film 12 with the gateinsulating film 13 being interposed therebetween.

A shape, a structure, and a size of the gate electrode 14 are notparticularly limited and may be appropriately selected depending on thepurpose. By way of example, in FIGS. 1A and 1B, the planar shape of thegate insulating film 13 is formed in a rectangular shape, with thelonger side being in the y-axis direction. The gate electrode 14substantially overlaps the gate insulating film 13 in a plan view.

A material of the gate electrode 14 is not particularly limited and maybe appropriately selected depending on the purpose. Examples of thematerial include metals such as aluminum (Al), platinum (Pt), palladium(Pd), gold (Au), silver (Ag), copper (Cu), zinc (Zn), nickel (Ni),chromium (Cr), tantalum (Ta), molybdenum (Mo), and titanium (Ti), alloysthereof, and mixtures of these metals.

Further, examples of the material of the gate electrode 14 includeconductive oxides such as indium oxide, zinc oxide, tin oxide, galliumoxide, and niobium oxide, complex compounds thereof, and mixturesthereof. Also, organic conductors such as polyethylene dioxythiophene(PEDOT) and polyaniline (PANI) may be used. An average thickness of thegate electrode 14 is not particularly limited and may be appropriatelyselected depending on the purpose, but is preferably 10 nm to 1 μm andmore preferably 50 nm to 300 nm.

The source electrode 15 and the drain electrode 16 are formed on thebase 11 and are in contact with the semiconductor film 12. The sourceelectrode 15 and the drain electrode 16 are formed to cover a part ofthe semiconductor film 12, and are formed spaced apart a predetermineddistance from each other, which serves as a channel region. The sourceelectrode 15 and the drain electrode 16 are electrodes that cause anelectric current to flow when a gate voltage is applied to the gateelectrode 14.

Shapes, structures, and sizes of the source electrode 15 and the drainelectrode 16 are not particularly limited and may be appropriatelyselected depending on the purpose. By way of example, in FIGS. 1A and1B, the planar shapes of the source electrode 15 and the drain electrode16 are formed in rectangular shapes, with the longer sides being in thex-axis direction.

Materials of the source electrode 15 and the drain electrode 16 are notparticularly limited and may be appropriately selected depending on thepurpose. Examples of the material include metals such as aluminum, gold,platinum, palladium, silver, copper, zinc, nickel, chromium, tantalum,molybdenum, and titanium, alloys thereof, and mixtures of these metals.In addition, conductive oxides such as indium oxide, zinc oxide, tinoxide, gallium oxide, and niobium oxide, complex compounds thereof, andmixtures thereof may be used. The source electrode 15 and the drainelectrode 16 may use a laminated structure of these materials.

An average thickness of the source electrode 15 and the drain electrode16 is not particularly limited and may be appropriately selecteddepending on the purpose. However, the average thickness of the sourceelectrode 15 and of the drain electrode 16 are formed smaller than theaverage thickness of the gate insulating film 13.

This prevents the source electrode 15 and the drain electrode 16 fromcoming in contact with the gate electrode 14. As a result, it ispossible to suppress a leakage current between the source electrode 15and the gate electrode 14 and also suppress a leakage current betweenthe drain electrode 16 and the gate electrode 14. Accordingly, favorabletransistor characteristics can be obtained.

The gate electrode covering layer 17 is formed in a predetermined regionon the gate electrode 14. The gate electrode covering layer 17 is formedin contact with the gate electrode 14 without making contact with otherelements constituting the field-effect transistor 10 including thesource electrode 15 and the drain electrode 16.

The gate electrode covering layer 17 is a layer formed of the samematerial as that of the source electrode 15 and the drain electrode 16,and has nearly the same thickness as that of the source electrode 15 andthe drain electrode 16. A combined planar shape of the source electrode15, the drain electrode 16, and the gate electrode covering layer 17 areformed in a rectangular shape, with the longer side being in the x-axisdirection. However, the source electrode 15, the drain electrode 16, andthe gate electrode covering layer 17 are spaced apart from one anotherand are not electrically connected to one another.

<Method for Producing Field-Effect Transistor>

Next, a method for producing the field-effect transistor illustrated inFIGS. 1A and 1B will be described. FIGS. 2A through 2D and FIG. 3Athrough 3C are diagrams illustrating a process for producing thefield-effect transistor of the first embodiment.

First, in a step illustrated in FIG. 2A, the base 11, which is a glassbase, for example, is prepared. The semiconductor film 12 is formed onthe entire surface of the base 11. The material and the thickness of thebase 11 can be appropriately selected as described above. Also, in orderto clean the surface of the base 11 and improve adhesiveness,pretreatments such as oxygen plasma, UV ozone, and UV radiation cleaningare preferably performed.

A method for forming the semiconductor film 12 is not particularlylimited and may be appropriately selected depending on the purpose.Examples of the method for forming the film include a vacuum processsuch as a sputtering method, a pulse laser deposition (PLD) method, achemical vapor deposition (CVD) method, and an atomic layer deposition(ALD) method, and also include a solution process such as a dip coatingmethod, a spin coating method, and a die coating method. The materialand the thickness of the semiconductor film 12 can be appropriatelyselected as described above.

After the semiconductor film 12 is formed, a resist made of aphotosensitive resin is formed on the entire surface of thesemiconductor film 12 and is subjected to an exposure and developmentprocess (photolithography process). As a result, a resist layer 300 (anetching mask) covering a predetermined region on the semiconductor film12 is formed.

Next, in a step illustrated in FIG. 2B, using the resist layer 300 asthe etching mask, a region of the semiconductor film 12 that is notcovered by the resist layer 300 is removed by etching. For example, thesemiconductor film 12 can be removed by wet etching.

Next, in a step illustrated in FIG. 2C, after the resist layer 300 isremoved, the gate insulating film 13 and the gate electrode 14, whichcover the semiconductor film 12, are sequentially laminated over theentire surface of the base 11.

A method for forming the gate insulating film 13 is not particularlylimited and may be appropriately selected depending on the purpose.Examples of the method for forming the film include a vacuum processsuch as a sputtering method, a pulse laser deposition (PLD) method, achemical vapor deposition (CVD) method, and an atomic layer deposition(ALD) method, and also include a solution process such as a dip coatingmethod, a spin coating method, and a die coating method. The materialand the thickness of the gate insulating film 13 can be appropriatelyselected as described above.

A method for forming the gate electrode 14 is not particularly limitedand may be appropriately selected depending on the purpose. Examplesinclude a vacuum process such as a sputtering method, a pulse laserdeposition (PLD) method, a chemical vapor deposition (CVD) method, andan atomic layer deposition (ALD) method, and also include a solutionprocess such as a dip coating method, a spin coating method, and a diecoating method. The material and the thickness of the gate electrode 14can be appropriately selected as described above.

After the gate insulating film 13 and the gate electrode 14 are formed,a resist made of a photosensitive resin is formed on the entire surfaceof the gate electrode 14 and is subjected to the exposure anddevelopment process (photolithography process). As a result, a resistlayer 310 (an etching mask) covering a predetermined region on the gateelectrode 14 is formed.

Next, in a step illustrated in FIG. 2D, using the resist layer 310 asthe etching mask, a region of the gate electrode 14 that is not coveredby the resist layer 310 is removed by etching. Subsequently, a region ofthe gate insulating film 13 that is not covered by the resist layer 310is removed by etching.

For example, when the gate electrode 14 is formed of Al, Mo, or an alloycontaining one of Al and Mo, the gate electrode 14 can be etched byusing a PAN (phosphoric-acetic-nitric-acid) based etching solution. ThePAN-based etching solution is a mixed solution of phosphoric acid,nitric acid, and acetic acid.

Also, when the gate insulating film 13 is an oxide film containing atleast the above-described Group A element and the Group B element, thegate insulating film 13 can be etched by using an etching solutioncontaining at least any one of hydrochloric acid, oxalic acid, nitricacid, phosphoric acid, acetic acid, sulfuric acid, and hydrogenperoxide.

Further, when the gate insulating film 13 is an oxide film containingSi, the gate insulating film 13 can be etched by using an etchingsolution containing at least any one of hydrofluoric acid, ammoniumfluoride, hydrogen fluoride ammonium, and organic alkali.

Moreover, the resist layer 310 has an etching resistance to PAN-basedetching solutions.

Accordingly, the gate electrode 14 and the gate insulating film 13 canbe etched by performing a single mask production process (namely, aprocess for forming the resist layer 310) only. For example, etching canbe performed by using the same mask (resist layer 310). Namely, unlikeconventional techniques, separate masks are not required to be producedfor etching of the gate electrode 14 and for etching of the gateinsulating film 13.

Next, in a step illustrated in FIG. 3A, after the resist layer 310 isremoved, the source electrode 15 and the drain electrode 16 covering thebase 11 and the semiconductor film 12 are formed with the gateinsulating film 13 being interposed between the source electrode 15 andthe drain electrode 16, such that a channel is formed in thesemiconductor film 12. At the same time, the gate electrode coveringlayer 17 is formed on the gate electrode 14.

A method for forming the source electrode 15, the drain electrode 16,and the gate electrode covering layer 17 is not particularly limited andmay be appropriately selected depending on the purpose. Examples of themethod include a method for forming a film by using a sputtering method,a vacuum deposition method, a dip coating method, a spin coating method,and a die coating method, and subsequently patterning the film byphotolithography. The material and the thickness of the source electrode15, the drain electrode 16, and the gate electrode covering layer 17 canbe appropriately selected as described above.

After the source electrode 15, the drain electrode 16, and the gateelectrode covering layer 17 are formed, a resist made of aphotosensitive resin is formed on the entire surface of the sourceelectrode 15, the drain electrode 16, and the gate electrode coveringlayer 17, and is subjected to the exposure and development process(photolithography process). As a result, a resist layer 320 (an etchingmask) covering predetermined regions on the source electrode 15, thedrain electrode 16, and the gate electrode covering layer 17 is formed.

Next, in a step illustrated in FIG. 3B, using the resist layer 320 asthe etching mask, regions of the source electrode 15 and the drainelectrode 16 that are not covered by the resist layer 320 are removed byetching. For example, the regions of the source electrode 15 and thedrain electrode 16 can be removed by wet etching. The gate electrodecovering layer 17 is completely covered by the resist layer 320. Thus,the gate electrode covering layer 17 is not etched.

Next, in a step illustrated in FIG. 3C, the resist layer 320 is removed.Accordingly, the self-aligned top-gate field-effect transistor 10 isproduced.

The field-effect transistor 10 of the first embodiment is formed suchthat the source electrode 15 and the drain electrode 16 are in contactwith the semiconductor film 12. Unlike conventional techniques, thefield-effect transistor 10 of the first embodiment does not require astructure in which a source electrode and a drain electrode formed on aninterlayer insulating layer are connected to a source region and a drainregion of a semiconductor film 12 through contact holes. In addition, animpurity region or the like is not required to be formed. Accordingly,the field-effect transistor 10 can be miniaturized.

Further, the field-effect transistor 10 is a self-aligned(self-alignment structure) field-effect transistor in which the sourceelectrode 15 and the drain electrode 16 are produced in a self-alignmentmanner by using the gate insulating film 13 as a mask. This allows achannel length to be controlled based on the width of the gateinsulating film 13, making it possible to miniaturize the field-effecttransistor 10.

Moreover, in the field-effect transistor 10, the planar shape of thegate insulating film 13 is substantially the same as the planar shape ofthe gate electrode 14. Therefore, parasitic capacitance can be reduced.As a result, switching characteristics of the field-effect transistor 10can be enhanced.

Furthermore, the thickness of the source electrode 15 and of the drainelectrode 16 are smaller than the thickness of the gate insulating film13. This prevents the source electrode 15 and the drain electrode 16from making contact with the gate electrode 14. Also, because the sourceelectrode 15 and the drain electrode 16 are thin, a difference in levelis formed between the source electrode 15 and the gate electrodecovering layer 17 and also between the drain electrode 16 and the gateelectrode covering layer 17. This ensures that the source electrode 15and the drain electrode 16 are separated from the gate electrodecovering layer 17. Accordingly, it is possible to suppress a leakagecurrent between the source electrode 15 and the gate electrode 14 andalso a leakage current between the drain electrode 16 and the gateelectrode 14. Therefore, favorable transistor characteristics can beobtained.

In addition, in the field-effect transistor 10, the gate electrode 14and the gate insulating film 13 are etched by using the same mask. Thisallows the number of etching masks used in the process for producing thefield-effect transistor 10 to be reduced as compared to conventionalproduction processes, making it possible to simplify the process forproducing the field-effect transistor 10.

Second Embodiment

A second embodiment illustrates an example in which a gate electrode isformed in an overhang shape. In the second embodiment, a description ofthe same elements as those of the above-described embodiment may beomitted.

FIG. 4 is a cross-sectional view illustrating a field-effect transistorof the second embodiment. A difference between a field-effect transistor10A illustrated in FIG. 4 and the field-effect transistor 10 (see FIG.1A) is that the gate electrode 14 is replaced with a gate electrode 14A.

The gate electrode 14A is formed in an overhang shape. Namely, a gateinsulating film 13 includes a region whose width is narrower than thegate electrode 14A.

In the example of FIG. 4, the sides of the gate electrode 14A areperpendicular to the upper surface of a base 11. The outer edge portionof the lower surface of the gate electrode 14A protrudes from theperiphery of the upper surface of the gate insulating film 13. Namely,in the entire region of the gate electrode 14A, the width of the gateelectrode 14A is wider than the width of the gate insulating film 13. Anoverhang amount (a difference in width between the gate electrode 14Aand the gate insulating film 13 illustrated in the cross section of FIG.4) can be set to approximately 100 nm to a few hundred nm, for example.

However, the sides of the gate electrode 14A are not required to beperpendicular to the upper surface of the base 11. The gate electrode14A may be formed in a downward tapered shape that becomes narrowertoward the gate insulating film 13 or may be formed in an upward taperedshape that becomes wider toward the gate insulating film 13. Namely, aslong as the gate insulating film 13 has a region whose width is narrowerthan the width of the gate electrode 14A, the gate insulating film 13may be formed in any shape.

The gate electrode 14A in the overhang shape can be produced bycontrolling a wet etching process in the step illustrated in FIG. 2D.Namely, by controlling the wet etching process, the gate insulating film13 having the region whose width is narrower than the width of the gateelectrode 14A can be produced.

In this way, the field-effect transistor 10A of the second embodimenthas a similar structure to that of the field-effect transistor 10 of thefirst embodiment. Therefore, the field-effect transistor 10A can beminiaturized.

Further, in the field-effect transistor 10A, the gate electrode 14A isformed in an overhang shape and the gate insulating film 13 has a regionwhose width is narrower than the width of the gate electrode 14A. Thisensures that the source electrode 15 and the drain electrode 16 areseparated from the gate electrode covering layer 17. Also, the thicknessof the source electrode 15 and of the drain electrode 16 are smallerthan the thickness of the gate insulating film 13. Because of thissynergistic effect, it is possible to suppress a leakage current betweenthe source electrode 15 and the gate electrode 14A and also a leakagecurrent between the drain electrode 16 and the gate electrode 14A.Therefore, favorable transistor characteristics can be obtained.

Third Embodiment

A third embodiment illustrates an example in which a gate electrode hasan undercut. In the third embodiment, a description of the same elementsas those of the above-described embodiments may be omitted.

<Structure of Field-Effect Transistor>

FIG. 5 is a cross-sectional view illustrating a field-effect transistorof the third embodiment. A difference between a field-effect transistor10B illustrated in FIG. 5 and the field-effect transistor 10 (see FIG.1A) is that the gate electrode 14 is replaced with a gate electrode 14B.

The gate electrode 14B has an undercut. Namely, the gate electrode 14Bincludes a region whose width is narrower than the width of a gateinsulating film 13.

In the example of FIG. 5, the gate electrode 14B is a multi-layer filmin which a conductive film 142 is laminated on a conductive film 141. Inthe multi-layer film constituting the gate electrode 14B, widths of thelayers become narrower layer by layer toward the gate insulating film13. To be more specific, the width of the conductive film 141 isnarrower than the width of the conductive film 142. Therefore, the outeredge portion of the lower surface of the conductive film 142 protrudesfrom the periphery of the upper surface of the conductive film 141.Also, the width of the conductive film 141 is narrower than the width ofthe gate insulating film 13. Therefore, the outer edge portion of theupper surface of the gate insulating film 13 protrudes from theperiphery of the lower surface of the conductive film 141.

An undercut amount (a difference in width between the conductive film141 and the conductive film 142 as illustrated in the cross section ofFIG. 5) can be set to approximately 100 nm to a few hundred nm, forexample.

A material of the conductive film 141 is not particularly limited andmay be appropriately selected depending on the purpose. For example, itis possible to use metals, alloys, mixtures of a plurality of metals,and conductive films other than metal films, which can be etched byusing an organic alkaline solution as an etching solution. Examples ofthe material include aluminum (Al), Al alloys (alloys mainly containingAl), and oxide films having conductivity.

Examples of the organic alkaline solution include strong alkalinesolutions such as tetramethyl ammonium hydroxide (TMAH-based),2-hydroxyethyl trimethylammonium hydroxide (CHOLINE-based), andmonoethanolamine solutions.

A material of the conductive film 142 is not particularly limited andmay be appropriately selected depending on the purpose. For example, itis possible to use metals, alloys, mixtures of a plurality of metals,and conductive films other than metal films, which have an etchingresistance to an organic alkaline solution and also have a higheretching rate for a predetermined etching solution than the conductivefilm 141. Examples of the material include metals such as molybdenum(Mo), tungsten (W), titanium (Ti), tantalum (Ta), chromium (Cr), copper(Cu), and nickel(Ni), alloys thereof, mixtures of these metals, andoxide films having conductivity.

An average thickness of the conductive film 141 is not particularlylimited and may be appropriately selected depending on the purpose, butis preferably 10 nm to 200 nm and more preferably 50 nm to 100 nm. Anaverage thickness of the conductive film 142 is not particularly limitedand may be appropriately selected depending on the purpose, but ispreferably 10 nm to 200 nm and more preferably 50 nm to 100 nm.

<Method for Producing Field-Effect Transistor>

In order to produce the field-effect transistor 10B, steps similar tothose described in FIG. 2A and FIG. 2B are performed first. Next, in astep illustrated in FIG. 6A, after a resist layer 300 is removed, a gateinsulating film 13 covering a semiconductor film 12 is formed over theentire surface of a base 11. Further, the conductive film 141 and theconductive film 142 are sequentially laminated on the gate insulatingfilm 13. A method for forming the gate insulating film 13 is asdescribed above.

A method for forming the conductive films 141 and 142 is notparticularly limited and may be appropriately selected depending on thepurpose. Examples of the method include a vacuum process such as asputtering method, a pulse laser deposition (PLD) method, a chemicalvapor deposition (CVD) method, and an atomic layer deposition (ALD)method, and also include a solution process such as a dip coatingmethod, a spin coating method, and a die coating method. Other examplesinclude a printing process such as an inkjet printing, a nanoimprinting,and a gravure printing.

As an example herein, a material (an Al alloy, for example) that can beetched by using an organic alkaline solution as an etching solution isselected as a material of the conductive film 141. A material (a Moalloy, for example) that has a higher etching rate for a predeterminedetching solution than the conductive film 141 is selected as a materialof the conductive film 142.

After the conductive film 142 is formed, a resist made of aphotosensitive resin is formed on the entire surface of the conductivefilm 142 and is subjected to the exposure and development process(photolithography process). As a result, a resist layer 310 (etchingmask) covering a predetermined region on the conductive film 142 isformed.

Next, in a step illustrated in FIG. 6B, using the resist layer 310 asthe etching mask, a region of the conductive film 142 that is notcovered by the resist layer 310 is removed by etching. By performingetching using an etching solution whose etching rate is higher for theconductive film 142 than for the conductive film 141, only the region ofthe conductive film 142 that is not covered by the resist layer 310 canbe removed by etching. At this time, the conductive film 141 is hardlyetched. The ratio of the etching rate of the conductive film 141 to theetching rate of the conductive film 142 is preferably at least 1:10.Further, the resist layer 310 has an etching resistance to the etchingsolution used in this step.

Next, in a step illustrated in FIG. 6C, a region of the conductive film141 that is not covered by the conductive film 142 is removed byetching. In this step, an organic alkaline solution is used as anetching solution. The resist layer 310 is soluble in the organicalkaline solution. Conversely, the conductive film 142 has an etchingresistance to the organic alkaline solution. Therefore, while the resistlayer 310 is dissolved, the conductive film 141 can be etched in adesired shape by using the conductive film 142 as a mask. Further,although the resist layer 310 is gradually dissolved, FIG. 6Cillustrates a state in which the resist layer 310 is completelydissolved. After the conductive film 141 is etched, the gate insulatingfilm 13 is etched by using the gate electrode 14B as a mask.

Moreover, in the step illustrated in FIG. 6C, the conductive film 142serves as an etching mask. Therefore, for example, after the stepillustrated in FIG. 6B is performed, the resist layer 310 may bepreliminarily removed by etching, and subsequently, the conductive film141 may be etched by using the conductive film 142 as the etching mask.

In the step illustrated in FIG. 6C, the width of the conductive film 141can be made narrower than the width of the conductive film 142 bycontrolling a wet etching process (such as an etching time). Namely, anundercut (a difference in width between the conductive film 141 and theconductive film 142 illustrated in the cross section of FIG. 6C) can beformed.

Accordingly, the gate electrode 14B and the gate insulating film 13 canbe etched by performing a single mask production process (namely, aprocess for forming the resist layer 310) only. Namely, unlikeconventional techniques, separate masks are not required to be producedfor etching of the gate electrode 14B and for etching of the gateinsulating film 13.

Herein, performing etching by a single mask production process may beexpressed as “etching using the same mask.” Namely, the expression“etching using the same mask” includes a case in which a plurality oflayers are etched by using the same resist layer as an etching mask, andalso includes a case in which a lower layer is etched by using an upperlayer as a mask when the resist layer is dissolved while the lower layeris being etched.

After the step illustrated in FIG. 6C, by performing steps similar tothose described in FIG. 3A through FIG. 3C, the self-aligned top-gatefield-effect transistor 10B illustrated in FIG. 5 is produced.

The field-effect transistor 10B of the third embodiment has a similarstructure to that of the field-effect transistor 10 of the firstembodiment. Therefore, the field-effect transistor 10B can beminiaturized.

Further, in the field-effect transistor 10B, the gate electrode 14B hasthe undercut. In a case where the source electrode 15, the drainelectrode 16, and the gate electrode covering layer 17 are formed bysputtering, sputter particles hardly reach the undercut portion. Thisensures that the source electrode 15 and the drain electrode 16 areseparated from the gate electrode covering layer 17. Also, the thicknessof the source electrode 15 and of the drain electrode 16 is smaller thanthe thickness of the gate insulating film 13. Because of thissynergistic effect, it is possible to suppress a leakage current betweenthe source electrode 15 and the gate electrode 14B and also a leakagecurrent between the drain electrode 16 and the gate electrode 14B.Therefore, favorable transistor characteristics can be obtained.

However, in the field-effect transistor 10B, the thickness of the sourceelectrode 15 and of the drain electrode 16 is not required to be smallerthan the thickness of the gate insulating film 13. In the field-effecttransistor 10B, the thickness of the source electrode 15 and of thedrain electrode 16 is smaller than the total thickness of the gateinsulating film 13 and the gate electrode 14B excluding the upper layer(namely, the thickness of the gate insulating film 13 plus the thicknessof the conductive film 141). This prevents the gate electrode 14B frommaking contact with the source electrode 15 and the drain electrode 16.

Fourth Embodiment

A fourth embodiment illustrates another example in which a gateelectrode has an undercut. In the fourth embodiment, a description ofthe same elements as those of the above-described embodiments may beomitted.

<Structure of Field-Effect Transistor>

FIG. 7 is a cross-sectional view illustrating a field-effect transistorof the fourth embodiment. A difference between a field-effect transistor10C illustrated in FIG. 7 and the field-effect transistor 10 (see FIG.1A) is that the gate electrode 14 is replaced with a gate electrode 14C.

The gate electrode 14C has an undercut. Namely, the gate electrode 14Chas a region whose width is narrower than the width of the gateinsulating film 13.

In the example of FIG. 7, the gate electrode 14C is a multi-layer filmin which a conductive film 142 and a conductive film 143 aresequentially laminated on a conductive film 141. In the multi-layer filmconstituting the gate electrode 14C, widths of the layers becomenarrower layer by layer toward a gate insulating film 13. To be morespecific, the width of the conductive film 141 is narrower than thewidth of the conductive film 142. Therefore, the outer edge portion ofthe lower surface of the conductive film 142 protrudes from theperiphery of the upper surface of the conductive film 141. Also, thewidth of the conductive film 142 is narrower than the width of theconductive film 143. Therefore, the outer edge portion of the lowersurface of the conductive film 143 protrudes from the periphery of theupper surface of the conductive film 142. Also, the width of theconductive film 141 is narrower than the width of the gate insulatingfilm 13. Therefore, the outer edge portion of the upper surface of thegate insulating film 13 protrudes from the periphery of the lowersurface of the conductive film 141.

An undercut amount (a difference in width between the conductive film141 and the conductive film 142 illustrated in the cross section of FIG.7) can be set to approximately 100 nm to a few hundred nm, for example.Also, an undercut amount (a difference in width between the conductivefilm 142 and the conductive film 143 illustrated in the cross section ofFIG. 7) can be set to approximately 100 nm to a few hundred nm, forexample.

The materials and the thicknesses of the conductive films 141 and 142are as described above. A material of the conductive film 143 is notparticularly limited and may be appropriately selected depending on thepurpose. For example, it is possible to use metals, alloys, mixtures ofa plurality of metals, and conductive films other than metal films,which have an etching resistance to an organic alkaline solution andalso have a higher etching rate for a predetermined etching solutionthan the conductive film 142. Examples of the material include metalssuch as molybdenum (Mo), tungsten (W), titanium (Ti), tantalum (Ta),chromium (Cr), copper (Cu), and nickel (Ni), alloys thereof, mixtures ofthese metals, and oxide films having conductivity. An average thicknessof the conductive film 143 is not particularly limited and may beappropriately selected depending on the purpose, but is preferably 10 nmto 200 nm and more preferably 50 nm to 100 nm.

<Method for Producing Field-Effect Transistor>

In order to produce the field-effect transistor 10C, steps similar tothose described in FIG. 2A and FIG. 2B of the first embodiment areperformed first. Next, in a step illustrated in FIG. 8A, after a resistlayer 300 is removed, a gate insulating film 13 covering a semiconductorfilm 12 is formed over the entire surface of a base 11. Further, theconductive film 141, the conductive film 142, and the conductive film143 are sequentially laminated on the gate insulating film 13. A methodfor forming the gate insulating film 13 is as described above. A methodfor forming the conductive film 143 can be the same as the method forforming the conductive films 141 and 142.

As an example herein, a material (an Al alloy, for example) that can beetched by using an organic alkaline solution as an etching solution isselected as a material of the conductive film 141. A material (a Moalloy, for example) that has an etching resistance to the organicalkaline solution and also has a higher etching rate for a predeterminedetching solution than the conductive film 141 is selected as a materialof the conductive film 142. Further, a material (Ti, for example) thathas an etching resistance to the organic alkaline solution and also hasa higher etching rate for a predetermined etching solution than theconductive film 142 is selected as a material of the conductive film143.

After the conductive film 143 is formed, a resist made of aphotosensitive resin is formed on the entire surface of the conductivefilm 143, and is subjected to the exposure and development process(photolithography process). As a result, a resist layer 310 (etchingmask) covering a predetermined region on the conductive film 143 isformed.

Next, in a step illustrated in FIG. 8B, using the resist layer 310 asthe etching mask, a region of the conductive film 143 that is notcovered by the resist layer 310 is removed by etching. By performingetching using an etching solution whose etching rate is higher for theconductive film 143 than for the conductive film 142, only the region ofthe conductive film 143 that is not covered by the resist layer 310 canbe removed by etching. At this time, the conductive film 142 is hardlyetched. The ratio of the etching rate of the conductive film 142 to theetching rate of the conductive film 143 is preferably at least 1:10.Further, the resist layer 310 has an etching resistance to the etchingsolution used in this step.

Next, in a step illustrated in FIG. 8C, using the resist layer 310 asthe etching mask, a region of the conductive film 142 that is notcovered by the resist layer 310 is removed by etching. By performingetching using an etching solution whose etching rate is higher for theconductive film 142 than for the conductive film 141, only the region ofthe conductive film 142 that is not covered by the resist layer 310 canbe removed by etching. At this time, the conductive film 141 is hardlyetched. The ratio of the etching rate of the conductive film 141 to theetching rate of the conductive film 142 is preferably at least 1:10.Further, the resist layer 310 has an etching resistance to the etchingsolution used in this step.

Next, in a step illustrated in FIG. 8D, a region of the conductive film141 that is not covered by the conductive films 142 and 143 is removedby etching. In this step, an organic alkaline solution is used as anetching solution. The resist layer 310 is soluble in the organicalkaline solution. Conversely, the conductive films 142 and 143 have anetching resistance to the organic alkaline solution. Therefore, whilethe resist layer 310 is dissolved, the conductive film 141 can be etchedin a desired shape by using the conductive films 142 and 143 as a mask.Further, although the resist layer 310 is gradually dissolved, FIG. 8Dillustrates a state in which the resist layer 310 is completelydissolved. After the conductive film 141 is etched, the gate insulatingfilm 13 is etched by using the gate electrode 14C as a mask.

Moreover, in the step illustrated in FIG. 8D, the conductive films 142and 143 serve as an etching mask. Therefore, for example, after the stepillustrated in FIG. 8B or FIG. 8C is performed, the resist layer 310 maybe preliminarily removed by etching, and subsequently, the conductivefilm 141 may be etched by using the conductive films 142 and 143 as anetching mask.

In the step illustrated in FIG. 8D, by controlling a wet etching process(such as an etching time), the width of the conductive film 142 can bemade narrower than the width of the conductive film 143, and further thewidth of the conductive film 141 can be made narrower than the width ofthe conductive film 142. Namely, an undercut (a difference in widthbetween the conductive film 141 and the conductive film 143 illustratedin the cross section of FIG. 8D) can be formed wider.

Accordingly, the gate electrode 14C and the gate insulating film 13 canbe etched by performing a single mask production process (namely, aprocess for forming the resist layer 310) only. Namely, unlikeconventional techniques, separate masks are not required to be producedfor etching of the gate electrode 14C and for etching of the gateinsulating film 13.

After the step illustrated in FIG. 8D, by performing steps similar tothose described in FIG. 3A through FIG. 3C, the self-aligned top-gatefield-effect transistor 10C illustrated in FIG. 7 is produced.

The field-effect transistor 10C of the fourth embodiment has a similarstructure to that of the field-effect transistor 10 of the firstembodiment. Therefore, the field-effect transistor 10C can beminiaturized.

Further, because the gate electrode 14C of the field-effect transistor10C has a three-layer structure, etching conditions of the layers can bemore easily adjusted than the gate electrode 14B having a two-layerstructure. Therefore, an undercut amount of the field-effect transistor10C can be increased further than that of the field-effect transistor10B. Accordingly, in a case where the source electrode 15, the drainelectrode 16, and the gate electrode covering layer 17 are formed bysputtering, sputter particles hardly reach the undercut portion.

This further ensures that the source electrode 15 and the drainelectrode 16 are separated from the gate electrode covering layer 17.Also, the thickness of the source electrode 15 and of the drainelectrode 16 is smaller than the thickness of the gate insulating film13. Because of this synergistic effect, it is possible to suppress aleakage current between the source electrode 15 and the gate electrode14B and also a leakage current between the drain electrode 16 and thegate electrode 14C. Therefore, favorable transistor characteristics canbe obtained.

However, in the field-effect transistor 10C, the thickness of the sourceelectrode 15 and of the drain electrode 16 is not required to be smallerthan the thickness of the gate insulating film 13. In the field-effecttransistor 10C, the thickness of the source electrode 15 and the drainelectrode 16 is smaller than the total thickness of the gate insulatingfilm 13 and the gate electrode 14C excluding the uppermost layer(namely, the thickness of the gate insulating film 13 plus the thicknessof the conductive film 141 plus the thickness of the conductive film142). This prevents the gate electrode 14C from making contact with thesource electrode 15 and the drain electrode 16.

Fifth Embodiment

A fifth embodiment illustrates an example of a gate electrode having atwo-layer structure in which an upper electrode layer has a narrowerpattern width than a pattern width of a lower electrode layer. In thefifth embodiment, a description of the same elements as those of theabove-described embodiments may be omitted.

<Structure of Field-Effect Transistor>

FIG. 9 is a cross-sectional view illustrating a field-effect transistorof the fifth embodiment. A difference between a field-effect transistor10D illustrated in FIG. 9 and the field-effect transistor 10 (see FIG.1A) is that the gate electrode 14 is replaced with a gate electrode 14D.

The gate electrode 14D has two electrode layers. In the example of FIG.9, the gate electrode 14D is a multi-layer film in which a conductivefilm 142 is laminated on a conductive film 141. In the multi-layer filmconstituting the gate electrode 14D, widths of the layers becomenarrower layer by layer toward a gate insulating film 13. To be morespecific, the width of the conductive film 141 is narrower than thewidth of the conductive film 142. Therefore, the outer edge portion ofthe upper surface of the conductive film 141 protrudes from theperiphery of the lower surface of the conductive film 142.

A material of the conductive film 141 is not particularly limited andmay be appropriately selected depending on the purpose. For example, itis possible to use metals, alloys, mixtures of a plurality of metals,and conductive films other than metal films, which can be etched byusing an organic alkaline solution as an etching solution. Examples ofthe material include aluminum (Al), Al alloys (alloys mainly containingAl), and oxide films having conductivity.

Examples of the organic alkaline solution include strong alkalinesolutions such as tetramethyl ammonium hydroxide (TMAH-based),2-hydroxyethyl trimethylammonium hydroxide (CHOLINE-based), andmonoethanolamine solutions.

A material of the conductive film 142 is not particularly limited andmay be appropriately selected depending on the purpose. For example, itis possible to use metals, alloys, mixtures of a plurality of metals,and conductive films other than metal films, which have an etchingresistance to an organic alkaline solution and also have a higheretching rate for a predetermined etching solution than the conductivefilm 141. Examples of the material include metals such as molybdenum(Mo), tungsten (W), titanium (Ti), tantalum (Ta), chromium (Cr), copper(Cu), and nickel (Ni), alloys thereof, mixtures of these metals, andoxide films having conductivity.

An average thickness of the conductive film 141 is not particularlylimited and may be appropriately selected depending on the purpose, butis preferably 10 nm to 200 nm and more preferably 50 nm to 100 nm. Anaverage thickness of the conductive film 142 is not particularly limitedand may be appropriately selected depending on the purpose, but ispreferably 10 nm to 200 nm and more preferably 50 nm to 100 nm.

<Method for Producing Field-Effect Transistor>

In order to produce the field-effect transistor 10D, steps similar tothose described in FIG. 2A and FIG. 2B of the first embodiment areperformed, and a resist layer 300 is removed. Next, in the stepillustrated in FIG. 6A, a gate insulating film 13 covering asemiconductor film 12 is formed over the entire surface of a base 11.Further, the conductive film 141 and the conductive film 142 aresequentially laminated on the gate insulating film 13. A method forforming the gate insulating film 13 is as described above.

A method for forming the conductive films 141 and 142 is notparticularly limited and may be appropriately selected depending on thepurpose. Examples of the method include a vacuum process such as asputtering method, a pulse laser deposition (PLD) method, a chemicalvapor deposition (CVD) method, and an atomic layer deposition (ALD)method, and also include a solution process such as a dip coatingmethod, a spin coating method, and a die coating method. Other examplesinclude a printing process such as an inkjet printing, a nanoimprinting,and a gravure printing.

As an example herein, a material (an Al alloy, for example) that can beetched by using an organic alkaline solution as an etching solution isselected as a material of the conductive film 141. A material (a Moalloy, for example) that has a higher etching rate for a predeterminedetching solution than the conductive film 141 is selected as a materialof the conductive film 142.

After the conductive film 142 is formed, a resist made of aphotosensitive resin is formed on the entire surface of the conductivefilm 142 and is subjected to the exposure and development process(photolithography process). As a result, a resist layer 310 (etchingmask) covering a predetermined region on the conductive film 142 isformed.

Next, in a step illustrated in FIG. 6B, using the resist layer 310 asthe etching mask, a region of the conductive film 142 that is notcovered by the resist layer 310 is removed by etching. By performingetching using an etching solution whose etching rate is higher for theconductive film 142 than for the conductive film 141, only the region ofthe conductive film 142 that is not covered by the resist layer 310 canbe removed by etching. At this time, the conductive film 141 is hardlyetched. The ratio of the etching rate of the conductive film 141 to theetching rate of the conductive film 142 is preferably at least 1:10.Further, the resist layer 310 has an etching resistance to the etchingsolution used in this step.

Next, in the step illustrated in FIG. 6C, a region of the conductivefilm 141 that is not covered by the conductive film 142 is removed byetching. In this step, an organic alkaline solution is used as anetching solution. The resist layer 310 is soluble in the organicalkaline solution. Conversely, the conductive film 142 has an etchingresistance to the organic alkaline solution. Therefore, while the resistlayer 310 is dissolved, the conductive film 141 can be etched in adesired shape by using the conductive film 142 as a mask. Further,although the resist layer 310 is gradually dissolved, FIG. 6Cillustrates the state in which the resist layer 310 is completelydissolved. After the conductive film 141 is etched, the gate insulatingfilm 13 is etched by using the gate electrode 14D as a mask.

Moreover, in the step illustrated in FIG. 6C, the conductive film 142serves as an etching mask. Therefore, for example, after the stepillustrated in FIG. 6B is performed, the resist layer 310 may bepreliminarily removed by etching, and subsequently, the conductive film141 may be removed by etching using the conductive film 142 as theetching mask.

Accordingly, the gate electrode 14D and the gate insulating film 13 canbe etched by performing a single mask production process (namely, aprocess for forming the resist layer 310) only. Namely, unlikeconventional techniques, separate masks are not required to be producedfor etching of the gate electrode 14D and for etching of the gateinsulating film 13.

After the step illustrated in FIG. 6C, by performing steps similar tothose described in FIG. 3A through FIG. 3C, the self-aligned top-gatefield-effect transistor 10D illustrated in FIG. 9 is produced.

The field-effect transistor 10D of the fifth embodiment has a similarstructure to that of the field-effect transistor 10 of the firstembodiment. Therefore, the field-effect transistor 10D can beminiaturized.

Furthermore, the thickness of the source electrode 15 and of the drainelectrode 16 are smaller than the thickness of the gate insulating film13. This prevents the source electrode 15 and the drain electrode 16from making contact with the gate electrode 14D. Also, because thesource electrode 15 and the drain electrode 16 are thin, a difference inlevel is formed between the source electrode 15 and the gate electrodecovering layer 17 and also between the drain electrode 16 and the gateelectrode covering layer 17. Accordingly, it is possible to suppress aleakage current between the source electrode 15 and the gate electrode14D and also a leakage current between the drain electrode 16 and thegate electrode 14D. Therefore, favorable transistor characteristics canbe obtained.

Sixth Embodiment

A sixth embodiment illustrates another example of a gate electrodehaving a three-layer structure in which a middle electrode layer has anundercut. In the sixth embodiment, a description of the same elements asthose of the above-described embodiments may be omitted.

<Structure of Field-Effect Transistor>

FIG. 10 is a cross-sectional view illustrating a field-effect transistorof the sixth embodiment. A difference between a field-effect transistor10E illustrated in FIG. 10 and the field-effect transistor 10 (see FIG.1A) is that the gate electrode 14 is replaced with a gate electrode 14E.

The gate electrode 14E has a three-layer structure in which a middleelectrode layer has an undercut. In the example of FIG. 10, the gateelectrode 14E is a multi-layer film in which a conductive film 142 and aconductive film 143 are sequentially laminated on a conductive film 141.In the multi-layer film constituting the gate electrode 14E, the widthof the conductive film 142 is narrower than the widths of the conductivefilm 141 and the conductive film 143.

An undercut amount (a difference in width between the conductive film142 and the conductive film 143 illustrated in the cross section of FIG.10) can be set to approximately 100 nm to a few hundred nm, for example.

The materials and the thicknesses of the conductive films 141 and 142are as described above. A material of the conductive film 143 is notparticularly limited and may be appropriately selected depending on thepurpose. For example, it is possible to use metals, alloys, mixtures ofa plurality of metals, and conductive films other than metal films,which have an etching resistance to an organic alkaline solution andalso have a higher etching rate for a predetermined etching solutionthan the conductive film 142. Examples of the material include metalssuch as molybdenum (Mo), tungsten (W), titanium (Ti), tantalum (Ta),chromium (Cr), copper (Cu), and nickel (Ni), alloys thereof, mixtures ofthese metals, and oxide films having conductivity. An average thicknessof the conductive film 143 is not particularly limited and may beappropriately selected depending on the purpose, but is preferably 10 nmto 200 nm and more preferably 50 nm to 100 nm.

<Method for Producing Field-Effect Transistor>

In order to produce the field-effect transistor 10E, steps similar tothose described in FIG. 2A and FIG. 2B of the first embodiment areperformed, and a resist layer 300 is removed. Next, in the stepillustrated in FIG. 8A, a gate insulating film 13 covering asemiconductor film 12 is formed over the entire surface of a base 11.Further, the conductive film 141, the conductive film 142, and theconductive film 143 are sequentially laminated on the gate insulatingfilm 13. A method for forming the gate insulating film 13 is asdescribed above. A method for forming the conductive film 143 can be thesame as the method for forming the conductive films 141 and 142.

As an example herein, a material (an Al alloy, for example) that can beetched by using an organic alkaline solution as an etching solution isselected as a material of the conductive film 141. A material (a Moalloy, for example) that has an etching resistance to the organicalkaline solution and also has a higher etching rate for a predeterminedetching solution than the conductive film 141 is selected as a materialof the conductive film 142. Further, a material (Ti, for example) thathas an etching resistance to the organic alkaline solution and also hasa higher etching rate for a predetermined etching solution than theconductive film 142 is selected as a material of the conductive film143.

After the conductive film 143 is formed, a resist made of aphotosensitive resin is formed on the entire surface of the conductivefilm 143, and is subjected to the exposure and development process(photolithography process). As a result, the resist layer 310 (etchingmask) covering a predetermined region on the conductive film 143 isformed.

Next, in the step illustrated in FIG. 8B, using the resist layer 310 asthe etching mask, a region of the conductive film 143 that is notcovered by the resist layer 310 is removed by etching. By performingetching using an etching solution whose etching rate is higher for theconductive film 143 than for the conductive film 142, only the region ofthe conductive film 143 that is not covered by the resist layer 310 canbe removed by etching. At this time, the conductive film 142 is hardlyetched. The ratio of the etching rate of the conductive film 142 to theetching rate of the conductive film 143 is preferably at least 1:10.Further, the resist layer 310 has an etching resistance to the etchingsolution used in this step.

Next, in a step illustrated in FIG. 8C, using the resist layer 310 asthe etching mask, a region of the conductive film 142 that is notcovered by the resist layer 310 is removed by etching. By performingetching using an etching solution whose etching rate is higher for theconductive film 142 than for the conductive film 141, only the region ofthe conductive film 142 that is not covered by the resist layer 310 canbe removed by etching. At this time, the conductive film 141 is hardlyetched. The ratio of the etching rate of the conductive film 141 to theetching rate of the conductive film 142 is preferably at least 1:10.Further, the resist layer 310 has an etching resistance to the etchingsolution used in this step.

Next, in the step illustrated in FIG. 8D, a region of the conductivefilm 141 that is not covered by the conductive films 142 and 143 isremoved by etching. In this step, an organic alkaline solution is usedas an etching solution. The resist layer 310 is soluble in the organicalkaline solution. Conversely, the conductive films 142 and 143 have anetching resistance to the organic alkaline solution. Therefore, whilethe resist layer 310 is dissolved, the conductive film 141 can be etchedin a desired shape by using the conductive films 142 and 143 as a mask.Further, although the resist layer 310 is gradually dissolved, FIG. 8Dillustrates a state in which the resist layer 310 is completelydissolved. After the conductive film 141 is etched, the gate insulatingfilm 13 is etched by using the gate electrode 14E as a mask.

Moreover, in the step illustrated in FIG. 8D, the conductive films 142and 143 serve as an etching mask. Therefore, for example, after the stepillustrated in FIG. 8B or FIG. 8C is performed, the resist layer 310 maybe preliminarily removed by etching, and subsequently, the conductivefilm 141 may be etched by using the conductive films 142 and 143 as anetching mask.

Accordingly, the gate electrode 14E and the gate insulating film 13 canbe etched by performing a single mask production process (namely, aprocess for forming the resist layer 310) only. Namely, unlikeconventional techniques, separate masks are not required to be producedfor etching of the gate electrode 14E and for etching of the gateinsulating film 13.

After the step illustrated in FIG. 8D, by performing steps similar tothose described in FIG. 3A through FIG. 3C, the self-aligned top-gatefield-effect transistor 10E illustrated in FIG. 10 is produced.

The field-effect transistor 10E of the sixth embodiment has a similarstructure to that of the field-effect transistor 10 of the firstembodiment. Therefore, the field-effect transistor 10E can beminiaturized.

Also, the thickness of the source electrode 15 and of the drainelectrode 16 is smaller than the thickness of the gate insulating film13. This prevents the gate electrode 14E from making contact with thesource electrode 15 and the drain electrode 16. Also, because the sourceelectrode 15 and the drain electrode 16 are thin, a difference in levelis formed between the source electrode 15 and the gate electrodecovering layer 17 and also between the drain electrode 16 and the gateelectrode covering layer 17. This ensures that the source electrode 15and the drain electrode 16 are separated from the gate electrodecovering layer 17. Accordingly, it is possible to suppress a leakagecurrent between the source electrode 15 and the gate electrode 14E andalso a leakage current between the drain electrode 16 and the gateelectrode 14E. Therefore, favorable transistor characteristics can beobtained.

Example 1

In Example 1, a top-gate field-effect transistor as illustrated in FIG.4 was produced by using the production process illustrated in FIGS. 2Athrough 2D and FIGS. 3A through 3C.

First, 0.1 mol (35.488 g) of indium nitrate (In(NO₃)₃.3H₂O) was weighedand was dissolved in 100 ml of ethylene glycol monomethyl ether toobtain solution A. Also, 0.02 mol (7.503 g) of aluminum nitrate(Al(NO3)₃.9H₂O) was weighed and was dissolved in 100 ml of ethyleneglycol monomethyl ether to obtain solution B. Further, 0.005 mol (1.211g) of rhenium oxide (Re₂O₇) was weighed and 500 ml of ethylene glycolmonomethyl ether was dissolved in 500 ml of ethylene glycol monomethylether to obtain solution C. Solution A (199.9 ml), solution B (50 ml),and solution C (10 ml), and 1,2-propanediol (420 ml) were mixed andstirred at room temperature to make a coating solution for producing an-type oxide semiconductor. Next, the above-described coating solutionfor producing the n-type oxide semiconductor was applied to the base 11by an inkjet printing method, and was baked at 300° C. for one hour atatmospheric pressure. The thickness of the resultant semiconductor film12 was 50 nm. Next, a resist layer 300 serving as a mask was formed onthe semiconductor film 12 and the semiconductor film 12 was patterned byphotolithography and etching.

Next, in 1 ml of toluene, 1.10 ml of Lanthanum 2-ethylhexanoate toluenesolution (a LA content of 7%, Wako 122-03371, available from WakoChemical Ltd.) and 0.30 ml of strontium 2-ethylhexanoate toluenesolution (a Sr content of 2%, Wako 195-09561, available from WakoChemical Ltd.) were mixed to obtain a coating solution for forming agate insulating film.

Next, 0.4 ml of the coating solution for forming the gate insulatingfilm was dropped and spin-coated on the base 11 and the semiconductorfilm 12 under predetermined conditions (spinning was performed at 500rpm for 5 seconds followed by 3,000 rpm for 20 seconds, and brought to astop at 0 rpm in 5 seconds). Next, the resultant film was dried at 120°C. for 1 hour at atmospheric pressure, baked at 400° C. for 3 hours inan O₂ atmosphere, and annealed at 50° C. for 1 hour at atmosphericpressure to form an oxide film as a gate insulating film 13. The averagethickness of the gate insulating film 13 was approximately 110 nm.

Next, as a gate electrode 14, an Al alloy film was formed on the gateinsulating film 13 by a sputtering method. Next, a resist layer 310serving as a mask was formed on the gate electrode 14. The gateinsulating film 13 and the gate electrode 14 were patterned byphotolithography and etching. At this time, an overhang shapeillustrated in FIG. 4 was formed by adjusting the etching process.

Next, as a source electrode 15 and a drain electrode 16, Al alloy filmswere formed by the sputtering method. A gate electrode covering layer 17made of the same material as the material of the source electrode 15 andthe drain electrode 16 and having nearly the same thickness as thethickness of the source electrode 15 and the drain electrode 16 wasformed on the gate electrode 14.

Next, a resist layer 320 serving as a mask was formed on the sourceelectrode 15, the drain electrode 16, and the gate electrode coveringlayer 17. The source electrode 15 and the drain electrode 16 werepatterned by photolithography and etching.

By removing the resist layer 320, a self-aligned top-gate field-effecttransistor was produced.

Example 2

In Example 2, a top-gate field-effect transistor as illustrated in FIG.4 was produced by the process illustrated in FIGS. 2A through 2D andFIGS. 3A through 3C in the same manner as Example 1, except that a Moalloy film was formed as a source electrode 15, a drain electrode 16,and a gate electrode covering layer 17 by the sputtering method.

Example 3

In Example 3, a top-gate field-effect transistor as illustrated in FIG.4 was produced by the process illustrated in FIGS. 2A through 2D andFIGS. 3A through 3C in the same manner as Example 1, except that Mg—Inbased oxide was formed as a semiconductor film 12 by the sputteringmethod.

To be more specific, an In-based oxide semiconductor film (semiconductorlayer) was formed on a base 11 made of glass by the sputtering method.

A polycrystalline sintered material having a composition of In₂MgO₄ wasused as a sputtering target. The ultimate vacuum in a sputter chamberwas set to 2×10⁻⁵ Pa. The flow rates of argon gas and oxygen gas usedduring sputtering were adjusted and the total pressure was set to 0.3Pa. By adjusting the flow rate of oxygen, the amount of oxygen in theoxide semiconductor film was controlled and the electron carrier densitywas also controlled. The thickness of the resultant oxide semiconductorfilm (semiconductor layer) was 50 nm.

Example 4

In Example 4, a top-gate field-effect transistor as illustrated in FIG.4 was produced by the process illustrated in FIGS. 2A through 2D andFIGS. 3A through 3C in the same manner as Example 1, except that a gateinsulating film 13 consisting of a SiO₂ film was formed by a CVD method.

Comparative Example 1

In Comparative Example 1, a top-gate field-effect transistor asillustrated in FIG. 4 was produced by the process illustrated in FIGS.2A through 2D and FIGS. 3A through 3C in the same manner as Example 1,except that the thickness of the source electrode 15, the drainelectrode 16, and the gate electrode covering layer 17 was formed largerthan the thickness of the gate insulating film 13.

Comparative Example 2

In Comparative Example 2, after a gate insulating film 13 was formed inthe same manner as Example 1, a first mask was formed on the gateinsulating film 13 and the gate insulating film 13 was patterned byphotolithography and etching. Next, after the first mask was removed anda gate electrode 14 was formed on the patterned gate insulating film 13in the same manner as Example 1, a second mask was formed on the gateelectrode 14 and the gate electrode 14 was patterned by photolithographyand etching. The other steps described in Example 1 were performed inaccordance with the production process illustrated in FIGS. 2A through2D and FIGS. 3A through 3C. Accordingly, a top-gate field-effecttransistor as illustrated in FIG. 4 was produced.

<Evaluation of Field-Effect Transistor>

The performance of the field-effect transistors obtained in Examples 1through 4 and Comparative Examples 1 and 2 was evaluated using asemiconductor parameter analyzer (B1500 semiconductor parameteranalyzer, available from Agilent Technologies). To be more specific,with a source-drain voltage (Vds) being set to 10 V and the gate voltage(Vg) being changed from −15V to +15V, a source-drain current (Ids) andgate current (Ig) leakage (Ig leakage) were measured to evaluatecurrent-voltage characteristics. Table 1 illustrates the evaluationresults, along with the number of masks used to produce the field-effecttransistors in the respective examples.

TABLE 1 COMPARATIVE COMPARATIVE EXAMPLE 1 EXAMPLE 2 EXAMPLE 3 EXAMPLE 4EXAMPLE 1 EXAMPLE 2 IG ACCEPTABLE ACCEPTABLE ACCEPTABLE ACCEPTABLE NOTACCEPTABLE LEAKAGE ACCEPTABLE NO. OF 3 3 3 3 3 4 MASKS

As illustrated in Table 1, in the field-effect transistors produced inExamples 1 through 4 and Comparative Example 2, values of the Ig leakagepresented no problem. However, in the field-effect transistor producedin Comparative Example 1, values of the Ig leakage exceeded anacceptable value. Also, in Comparative Example 2, although the value ofthe Ig leakage presented no problem, four masks were required. Ascompared to Examples 1 through 4 in which the number of masks used wasthree, the production process of the field-effect transistor inComparative Example 2 was complicated and thus was unfavorable.

Further, the results of the performance evaluation of the transistorspresented that the insulation was maintained and favorable transistorcharacteristics were obtained as illustrated in FIG. 11. Although FIG.11 illustrates characteristics of the field-effect transistor producedin Example 1, the field-effect transistors produced in Examples 2through 4 presented substantially the same characteristics.

Seventh Embodiment

A seventh embodiment illustrates an example of a display element usingthe field-effect transistor of the first embodiment, a display device,and a system. In the seventh embodiment, a description of the sameelements as those of the above-described embodiment may be omitted.

(Display Element)

The display element of the seventh embodiment at least includes a lightcontrol element and a driving circuit configured to drive the lightcontrol element. The display element further includes other members asnecessary. The light control element is not particularly limited and maybe appropriately selected depending on the purpose as long as the lightcontrol element is an element configured to control light output inaccordance with a driving signal. Examples of the light control elementinclude an electroluminescent (EL) element, an electrochromic (EC)element, a liquid crystal element, an electrophoretic element, and anelectrowetting element.

The driving circuit is not particularly limited and may be appropriatelyselected depending on the purpose. Other members are not particularlylimited and may be appropriately selected depending on the purpose.

Because the display element of the seventh embodiment has thefield-effect transistor of the first embodiment, the field-effecttransistor can be miniaturized. Accordingly, the display element can bedownsized.

Also, in the field-effect transistor of the first embodiment, becausethe parasitic capacitance can be reduced, the switching characteristicscan be improved, and also because the leakage current can be suppressed,favorable transistor characteristics can be provided. Accordingly, thedisplay element of the seventh embodiment has high display qualities.

(Display Device)

The display device of the seventh embodiment at least includes aplurality of display elements of the seventh embodiment, a plurality ofwires, and a display control unit. The display device further includesother members as necessary. The plurality of display elements are notparticularly limited and may be appropriately selected depending on thepurpose, as long as the plurality of display elements are the displayelements of the seventh embodiment arranged in a matrix form.

The plurality of wires are not particularly limited and may beappropriately selected depending on the purpose, as long as theplurality of wires are capable of individually applying a gate voltageand supplying an image data signal to the field-effect transistors inthe plurality of display elements.

The display control unit is not particularly limited and may beappropriately selected depending on the purpose, as long as the displaycontrol is capable of individually controlling the gate voltage and thesignal voltage of the field-effect transistor via the plurality of wiresbased on the image data. Other members are not particularly limited andmay be appropriately selected depending on the purpose.

Because the display device of the seventh embodiment includes thefield-effect transistor of the first embodiment, the display device candisplay high-quality images.

(System)

The system of the seventh embodiment at least includes the displayapparatus of the seventh embodiment and an image data generating device.The image data generating device generates image data based on imageinformation to be displayed and outputs the image data to the displaydevice.

Because the system includes the display device according to the seventhembodiment, high-definition image information can be displayed.

The display element, the display device, and the system of the seventhembodiment will be specifically described below.

FIG. 12 illustrates a schematic block configuration of the televisionapparatus of the seventh embodiment. Connecting lines illustrated inFIG. 12 are for illustrating a flow of typical signals and information,and are not for illustrating the entire connection relationship betweenthe blocks.

A television apparatus 500 of the seventh embodiment includes a maincontroller 501, a tuner 503, an analog-digital converter (ADC) 504, ademodulating circuit 505, a transport stream (TS) decoder 506, an audiodecoder 511, a digital-to-analog (DA) converter (DAC) 512, an audiooutput circuit 513, a speaker 514, a video decoder 521, a video/OSDsynthesizing circuit 522, a video output circuit 523, a display device524, an OSD rendering circuit 525, a memory 531, an operating device532, a drive interface (drive IF) 541, a hard disk drive 542, an opticaldisc drive 543, an IR photodetector 551, a communication controller 552,and the like.

The main controller 501 controls the entire television apparatus 500,and includes a CPU, flash ROM, RAM, and the like. The flash ROM stores aprogram written in code that can be decoded by the CPU, and also storesvarious types of data used for processing by the CPU. The RAM is workingmemory.

The tuner 503 selects a preset channel from broadcast waves received byan antenna 610. The ADC 504 converts an output signal (analoginformation) of the tuner 503 to digital information. The demodulatingcircuit 505 demodulates the digital information from the ADC 504.

The TS decoder 506 decodes an output signal from the demodulatingcircuit 505 and separates the output signal into sound information andvideo information. The audio decoder 511 decodes the sound informationfrom the TS decoder 506. The DA converter (DAC) 512 converts an outputsignal from the audio decoder 511 to an analog signal.

The audio output circuit 513 outputs the output signal from the DAconverter (DAC) 512 to the speaker 514. The video decoder 521 decodesthe video information from the TS decoder 506. The video-OSDsynthesizing circuit 522 synthesizes an output signal from the videodecoder 521 and an output signal from the OSD rendering circuit 525.

The video output circuit 523 outputs an output signal from the video-OSDsynthesizing circuit 522 to the display device 524. The OSD renderingcircuit 525 includes a character generator for displaying characters andgraphics on a screen of the display device 524. Also, the OSD renderingcircuit 525 generates a signal including display information accordingto instructions from the operating device 532 and the IR photodetector551.

The memory 531 temporarily stores audio-visual (AV) data and other data.The operating device 532 includes an input medium (not illustrated) suchas a control panel, and indicates various types of information input bya user to the main controller 501. The drive IF 541 is an interactivecommunication interface. For example, the drive IF 541 is compatiblewith the ATAPI (AT attachment packet interface).

The hard disk drive 542 includes a hard disk and a driving deviceconfigured to drive the hard disk. The driving device records data onthe hard disk and reproduces the data recorded on the hard disk. Theoptical disc drive 543 records data on an optical disc (a DVD, forexample) and reproduces the data recorded on the optical disc.

The IR photodetector 551 receives a photosignal from a remote controltransmitter 620, and notifies the photosignal to the main controller501. The communication controller 552 controls communication with theInternet. Various types of information can be obtained via the Internet.

As illustrated in FIG. 13 by way of example, the display device 524includes a display unit 700 and a display control unit 780. Asillustrated in FIG. 14 by way of example, the display unit 700 includesa display 710 in which a plurality of display elements 702 are arrangedin a matrix form (herein, n×m number of display elements).

Also, as illustrated in FIG. 15 by way of example, the display 710includes n number of scanning lines (X0, X1, X2, X3, . . . , Xn-2, Xn-1)arranged along the x-axis direction at regular intervals, m number ofdata lines (Y0, Y1, Y2, Y3, . . . , Ym-1) arranged along the y-axisdirection at regular intervals, and m number of current supply linesarranged along the y-axis direction at regular intervals (Y0 i, Y1 i, Y2i, Y3 i, . . . , Ym-1 i). The display elements 702 can be identified bythe scanning lines and the data lines.

As illustrated in FIG. 16 by way of example, the respective displayelements 702 include an organic EL (electroluminescent) element 750 anda driving circuit 720 configured to cause the organic EL(electroluminescent) element 750 to emit light. Namely, the display 710is an organic EL display of what is known as an active matrix system.Also, the display 710 is a 32-inch color display, but the size of thedisplay 710 is not limited thereto.

As illustrated in FIG. 17 by way of example, the organic EL element 750includes an organic EL thin film layer 740, a cathode 712, and an anode714.

For example, the organic EL element 750 can be disposed next to thefield-effect transistor. In this case, the organic EL element 750 andthe field-effect transistor can be formed on the same base. However, thepresent invention is not limited thereto. For example, the organic ELelement 750 may be disposed above the field-effect transistor. In thiscase, the gate electrode is required to have transparency. Therefore, atransparent oxide having conductivity, such as ITO, In₂O₃, SnO₂, ZnO,Ga-added ZnO, Al-added ZnO, and Sb-added SnO₂, is used for the gateelectrode. In the organic EL element 750, aluminum (Al) is used for thecathode 712. Also, a magnesium (Mg)-silver (Ag) alloy, an aluminum(Al)-lithium (Li) alloy, indium tin oxide (ITO), and the like may beused. ITO is used for the anode 714. Further, an oxide havingconductivity such as In₂O₃, SnO₂, and ZnO and a silver (Ag)-neodymium(Nd) alloy may be used.

The organic EL thin film layer 740 includes an electron transportinglayer 742, a light emitting layer 744, and a hole transporting layer746. The cathode 712 is connected to the electron transporting layer742. The anode 714 is connected to the hole transporting layer 746. Whena predetermined voltage is applied between the anode 714 and the cathode712, the light emitting layer 744 emits light.

Also, as illustrated in FIG. 16, the driving circuit 720 includes twofield-effect transistors 810 and 820 and a capacitor 830. Thefield-effect transistor 810 operates as a switching element. A gateelectrode G is connected to a predetermined scanning line and a sourceelectrode S is connected to a predetermined data line. Also, a drainelectrode D is connected to one terminal of the capacitor 830.

The capacitor 830 is configured to store the state, namely data, of thefield-effect transistor 810. The other terminal of the capacitor 830 isconnected to a predetermined current supply line.

The field-effect transistor 820 is configured to supply a large currentto the organic EL element 750. A gate electrode G is connected to thedrain electrode D of the field-effect transistor 810. A drain electrodeD is connected to the anode 714 of the organic EL element 750. A sourceelectrode S is connected to a predetermined current supply line.

When the field-effect transistor 810 is turned on, the organic ELelement 750 is driven by the field-effect transistor 820.

As illustrated in FIG. 18 by way of example, the display control unit780 includes an image data processing circuit 782, a scanning linedriving circuit 784, and a data line driving circuit 786.

The image data processing circuit 782 determines brightness of aplurality of display elements 702 in the display 710 based on an outputsignal from the video output circuit 523. The scanning line drivingcircuit 784 individually applies a voltage to n number of scanning linesin accordance with an instruction from the image data processing circuit782. The data line driving circuit 786 individually applies a voltage tom number of data lines in accordance with an instruction from the imagedata processing circuit 782.

As is clear from the above description, in the television apparatus 500of the present embodiment, the video decoder 521, the video-OSDsynthesizing circuit 522, the video output circuit 523, and the OSDrendering circuit 525 constitute the image data generating device.

Although a case where the light control element is an organic EL elementhas been described above, the light control element is not limitedthereto and may be a liquid crystal element, an electrochromic element,an electrophoretic element, or an electrowetting element.

For example, when the light control element is a liquid crystal element,a liquid crystal display is used as the above-described display 710. Inthis case, as illustrated in FIG. 19, a current supply line is notrequired for the display element 703.

Further, in this case, as illustrated in FIG. 20 by way of example, adriving circuit 730 can be formed by a single field-effect transistor840, which is similar to the field-effect transistors (810 and 820)illustrated in FIG. 14. In the field-effect transistor 840, a gateelectrode G is connected to a predetermined scanning line and a sourceelectrode S is connected to a predetermined data line. Also, a drainelectrode D is connected to a pixel electrode and a capacitor 760 of aliquid crystal element 770. Reference numerals 762 and 772 in FIG. 20are counter electrodes (common electrodes) of the capacitor 760 and theliquid crystal element 770, respectively.

Moreover, instead of the field-effect transistor of the firstembodiment, the driving circuit may include any of the field-effecttransistors of the second through fourth embodiments.

Although a case where the system is a television apparatus has beendescribed in the above embodiments, the system of the present inventionis not limited thereto. Namely, the system is not limited as long as thesystem includes the display device 524 as a device configured to displayimages and information. For example, the system may be a computer system(including a personal computer) in which a computer is connected to thedisplay device 524.

Also, the display device 524 can be used as a display part in mobileinformation devices such as mobile phones, portable music players,portable video players, electronic books, personal digital assistants(PDAs) and in image devices such as still cameras and video cameras.Further, the display device 524 can be used as a display part fordisplaying various information in transportation systems such as cars,aircraft, trains, and ships. Further, the display device 524 can be usedas a display part for displaying various information in measuringdevices, analysis devices, medical equipment, and advertising media.

Although the present invention has been described with reference toembodiments, the present invention is not limited to these embodiments.Various variations and modifications may be made without departing fromthe scope of the invention as set forth in the accompanying claims.

The present application is based on Japanese priority application No.2017-053733, filed on Mar. 17, 2017, and Japanese priority applicationNo. 2018-045946, filed on Mar. 13, 2018, with the Japanese PatentOffice, the entire content of which is hereby incorporated by reference.

REFERENCE SIGNS LIST

-   -   10, 10A, 10B, 10C field-effect transistor    -   11 base    -   12 semiconductor film    -   13 gate insulating film    -   14, 14A, 14B, 14C gate electrode    -   15 source electrode    -   16 drain electrode    -   17 gate electrode covering layer    -   141, 142, 143 conductive film

1. A field-effect transistor comprising: a semiconductor film formed ona base; a gate insulating film formed on a part of the semiconductorfilm; a gate electrode formed on the gate insulating film; and a sourceelectrode and a drain electrode formed in contact with the semiconductorfilm, wherein a thickness of the source electrode and the drainelectrode is smaller than a thickness of the gate insulating film, andthe gate insulating film includes a region that is not in contact withthe source electrode or the drain electrode.
 2. The field-effecttransistor according to claim 1, wherein the field-effect transistor isa top-gate field-effect transistor.
 3. The field-effect transistoraccording to claim 1, wherein the gate insulating film includes a regionwhose width is narrower than a width of the gate electrode.
 4. Thefield-effect transistor according to claim 1, wherein the gate electrodeincludes a plurality of layers.
 5. The field-effect transistor accordingto claim 4, wherein widths of the plurality of layers become narrowerlayer by layer toward the gate insulating film.
 6. A field-effecttransistor comprising: a semiconductor film formed on a base; a gateinsulating film formed on a part of the semiconductor film; a gateelectrode including a plurality of layers formed on the gate insulatingfilm; and a source electrode and a drain electrode formed in contactwith the semiconductor film, wherein widths of the plurality of layersbecome narrower layer by layer toward the gate insulating film, athickness of the source electrode and the drain electrode is smallerthan a total thickness of the gate insulating film and the gateelectrode excluding an uppermost layer, and the gate insulating filmincludes a region that is not in contact with the source electrode orthe drain electrode.
 7. The field-effect transistor according to claim1, comprising a conductive film formed on the gate electrode and made ofthe same material as a material of the source electrode and the drainelectrode.
 8. The field-effect transistor according to claim 1, whereinthe semiconductor film includes an oxide semiconductor.
 9. A displayelement comprising: a driving circuit; and a light control elementconfigured to control light output in accordance with a driving signalfrom the driving circuit, wherein the light control element is driven bythe field-effect transistor according to claim
 1. 10. The displayelement according to claim 9, wherein the light control element is anelectroluminescent element, an electrochromic element, a liquid crystalelement, an electrophoretic element, or an electrowetting element.
 11. Adisplay device comprising: a display unit in which a plurality ofdisplay elements are arranged, each of the plurality of display elementsbeing the display element according to claim 9, and a display controlunit configured to control the plurality of display elementsindividually.
 12. A system comprising: the display device according toclaim 11; and an image data generating device configured to supply imagedata to the display device.
 13. A method for producing a field-effecttransistor, comprising: forming a semiconductor film on a base; forminga gate insulating film on a part of the semiconductor film; forming agate electrode on the gate insulating film; patterning the gateelectrode and the gate insulating film by etching using a same mask; andforming a source electrode and a drain electrode in contact with thesemiconductor film, wherein, in the forming of the source electrode andthe drain electrode, the source electrode and the drain electrode areformed such that a thickness of the source electrode and the drainelectrode is smaller than a thickness of the gate insulating film, andthe gate insulating film has a region that is not in contact with eitherthe source electrode or the drain electrode.
 14. The method forproducing a field-effect transistor according to claim 13, wherein, inthe forming of the source electrode and the drain electrode, aconductive film made of the same material as a material of the sourceelectrode and the drain electrode is formed on the gate electrode. 15.The method for producing a field-effect transistor according to claim13, wherein the gate electrode includes a plurality of conductive films,in the forming of the source electrode and the drain electrode, theplurality of conductive films are laminated on the gate insulating film,and in the patterning, etching is performed such that widths of theplurality of conductive films become narrower layer by layer toward thegate insulating film.
 16. The field-effect transistor according to claim6, comprising a conductive film formed on the gate electrode and made ofthe same material as a material of the source electrode and the drainelectrode.
 17. The field-effect transistor according to claim 6, whereinthe semiconductor film includes an oxide semiconductor.